As the complexity of implantable medical devices increases over time, telemetry systems for enabling such implantable medical devices to communicate with external communication devices, e.g., programmers, has become more important. For example, it is desirable for a physician to non-invasively exercise some amount of control over the implantable medical device, e.g., to turn the device on or off after implantation, to adjust various parameters of the implantable medical device after implantation, etc.
Further, as implantable medical devices include more advanced features, it is typically necessary to convey correspondingly more information to the implantable medical device relating to the selection and control of such advanced features. For example, if a pacemaker is selectively operable in various pacing modes, it is desirable that the physician be able to non-invasively select a mode of operation. Further, for example, if a pacemaker is capable of pacing at various rates, or of delivering stimulating pulses of varying energy levels, it is desirable that the physician be able to select, on a patient-by-patient basis, appropriate values for such variable operational parameters. Various types of information are conveyed to implanted medical devices by telemetry systems. For example, information conveyed to pacemakers may include, but is clearly not limited to, pacing modes, multiple rate response settings, electrode polarity, maximum and minimum pacing rates, output energy such as output pulse width and/or output current, sense amplifier sensitivity, refractory periods, calibration information, and rate response attack (acceleration) and decay (deceleration).
Not only has the complexity of implantable medical devices led to the need to convey correspondingly more information to the implantable medical device, but it has also become desirable to enable the implanted medical device to communicate information outside of the patient to an external communication device, e.g., programmer. For example, for diagnostic purposes it is desirable for the implanted device to be able to communicate information regarding its operational status to the physician. Various implantable medical devices are available which can transmit information to an external communication device, such as the transmission of a digitized ECG signal for display, storage, and/or analysis by the external communication device.
As used herein, the term "uplink" and "uplink telemetry" will be used to denote the communications channel for conveying information from the implanted medical device to an external communication device, e.g., a programmer. Conversely, the term "downlink" and "downlink telemetry" will be used to denote the communications channel for conveying information from an external communication device to the implanted medical device.
Various telemetry systems for providing the necessary communication channels between an external communication device and an implanted medical device have been described. For example, various telemetry systems are disclosed in the references listed in Table 1 below.
Typically, telemetry systems such as those described in Table 1 are employed in conjunction with an external programming/processing unit, e.g., an external communication device. A programmer for non-invasively programming a cardiac pacemaker is described in the following U.S. Patents to Hartlaub et al., each commonly assigned to the assignee of the present invention: U.S. Pat. No. 4,250,884, entitled "Apparatus For and Method Of Programming the Minimum Energy Threshold for Pacing Pulses to be Applied to a Patient's Heart;" U.S. Pat. No. 4,273,132, entitled "Digital Cardiac Pacemaker with Threshold Margin Check;" U.S. Pat. No. 4,273,133, entitled "Programmable Digital Cardiac Pacemaker with Means to Override Effects of Reed Switch Closure;" U.S. Pat. No. 4,233,985, entitled "Multi-Mode Programmable Digital Cardiac Pacemaker;" U.S. Pat. No. 4,253,466, entitled "Temporary and Permanent Programmable Digital Cardiac Pacemaker;" and U.S. Pat. No. 4,401,120, entitled "Digital Cardiac Pacemaker with Program Acceptance Indicator." Aspects of the programmer that are the subject of the foregoing Hartlaub et al. patents are also described in U.S. Pat. No. 4,208,008 to Smith, entitled "Pacing Generator Programming Apparatus Including Error Detection Means," and in U.S. Pat. No. 4,236,524 to Powell et al., entitled "Program Testing Apparatus."
Most commonly, telemetry systems for implantable medical devices employ a radio frequency (RF) transmitter and receiver in the implantable medical device, and a corresponding RF transmitter and receiver in the external communication device, e.g., programming unit. Within the implantable medical device, the transmitter and receiver use a an antenna for receiving downlink telemetry signals and for radiating RF signals for uplink telemetry. Specifically, the radiating RF signals are magnetically coupled through inductive (antenna) coils.
To communicate digital data using RF telemetry, a digital encoding scheme such as described in U.S. Pat. No. 5,127,404 to Wyborny et al., entitled "Improved Telemetry Format," is used. In particular, for downlink telemetry a pulse interval modulation scheme may be employed, wherein the external communication device, e.g., programmer, transmits a series of short RF "bursts" or pulses in which the duration of an interval between successive pulses, e.g., the interval from the trailing edge of one pulse to the trailing edge of the next pulse, encodes the data. For example, a shorter interval may encode a "0" bit while a longer interval may encode a "1" bit.
For uplink telemetry, pulse position modulation may be employed to encode uplink telemetry data. For pulse position modulation, a plurality of timeslots are defined in a data frame, and the presence or absence of pulses transmitted during each timeslot encodes the data. For example, a sixteen position data frame may be defined, wherein a pulse in one of the timeslots represents a unique four bit portion of data.
Programming devices typically interface with the implanted medical device through the use of a programming head or paddle. For example, generally, the programming head or paddle is a handheld unit adapted to be placed on or near the patient's body over the implant site of the patient's implanted medical device. The programming head may effect closure of a reed switch in the implantable medical device using a magnet to initiate a telemetry session. Thereafter, uplink and downlink communication may take place between the implanted medical device's transmitter/receiver and the receiver/transmitter of the external communication device. Other methods of initiating a telemetry session may also be used. For example, a wake-up pulse from an external communication device may be used to wake up the implanted medical device which polls its downlink receiver at an appropriate interval.
For programming arrangements, both uplink and downlink telemetry signal strength vary as a function of programming head positioning. Therefore, it is important for the programming head to be properly positioned over the patient's implant site so that downlink RF signals can be detected in the implantable medical device and uplink signals can be detected by the programming head. For example, if the programming head is too far away from the implantable medical device, the attenuation of RF signals transmitted across the boundary of the patient's skin may be too great, preventing the telemetry link from being established. Often, medical device programmers, for example, such as the Model 9710 or 9760 programmers commercially-available from Medtronic, Inc., are provided with a head positioning indicator, e.g., an audible or visible indicator, for indicating to a physician when the programming head is properly located over a patient's implanted medical device.
Conventionally, the technique used for determining when the programming head is properly positioned can be characterized as an "open loop" technique in that the determination of correct head positioning is based solely upon an assessment of whether the uplink signal (i.e., the signal transmitted from the implanted medical device to the external communication device) meets some minimum requirement. In such an open loop verification system, adequate downlink signal strength is not tested. For example, an open loop system for determining the proper positioning of a programming head is described in U.S. Pat. No. 4,531,523 to Anderson, entitled "Digital Gain Control for the Reception of Telemetry Signals from Implanted Medical Devices."
When downlink signal strength is not tested, it is important for the physician to be able to otherwise verify that programming RF signals transmitted from an external communication device are accurately received and processed by the implanted medical device. For example, circuitry in the implanted medical device may perform several different checks on the detected downlink telemetry signals, e.g., a parity check, and issue an acceptance signal if the downlink telemetry signals are found to be valid.
A communication protocol using handshaking can also be used to verify that a minimum downlink field strength for detection in the implanted medical device exists to signal a physician that correct head positioning has been achieved. However, conventional handshaking protocols do not provide any information useful for optimization of head positioning to ensure an adequate operating margin. In other words, proper programming head positioning may be indicated even though the programming head is actually marginally positioned, such that a very slight shift in positioning (e.g., due to patient motion) results in downlink telemetry failure.
One possible way to ensure an adequate margin between the strength of the downlink signals detected in an implanted medical device and the device's detection threshold (i.e., threshold below which detection does not occur) is to transmit downlink telemetry signals having much larger than nominal amplitudes. If extremely strong downlink signals are transmitted, the programmer could be assured that signals will be strong enough to exceed the detection threshold and be detected by the implanted medical device.
However, there are various disadvantages associated with excessively strong downlink telemetry signals. First, while power consumption is not a crucial factor in line-powered external communication devices, it is common for many programming devices to be portable and battery-powered so they are easily transported and can be used in a variety of clinical and/or non-clinical settings. For example, battery-powered programmers can be used by patients away from the clinical setting. It would be inefficient and undesirable to consume the limited battery power available for such devices with unnecessarily high energy downlink signals.
Perhaps a far greater disadvantage of transmitting high energy downlink signals is the possibility that the large RF energy bursts in the downlink transmission may interfere with the operation of the implanted medical device. Such interference may take various forms depending upon the implanted medical device. For example, magnetic field coupling into the lead system of a pacemaker during programming may occur. In other words, with use of high energy downlink telemetry bursts or pulses, it is possible for the downlink signals to induce voltages on implanted pace/sense leads. Such induced voltages may be interpreted by the implanted medical device's sensing circuitry as cardiac events and may thereby cause pacemaker inhibition. Further, such misinterpretation of cardiac events may lead to loss of synchronization with intrinsic cardiac activity.
Further, for example, excessively strong downlink telemetry signals have an electric field component that may cause misinterpreted signals to be provided to external communication devices. For example, various programmers provide ECG circuitry for detection of pacing pulses, and various ECG monitors are designed to process pacing artifacts. A strong downlink telemetry signal provides an electric field component that may be coupled onto a patient's skin during programming and handshake telemetry. Such coupling may undesirably present artifacts causing uncertainty when using skin electrodes to sense pacing pulses, e.g., false sensing of pacing pulses.
Furthermore, it is clinically possible for a patient to have more than one implanted device. If devices are implanted too close to one another, unintended communication to an adjacent device may become possible with excessive downlink energy.
U.S. Pat. No. 5,324,315 to Grevious, entitled "Closed-Loop Downlink Telemetry and Method for Implantable Medical Device," describes a closed-loop system in which one or more of the problems described above are addressed. In the system of U.S. Pat. No. 5,324,315, a specific type of downlink telemetry pulse is transmitted from the external communication device to the implanted medical device. In particular, the downlink pulses are RF bursts having a linear ramping envelope. The characteristics of the downlink burst envelope are such that the amplitude of the signal as detected by the implanted medical device's receiver, relative to the receiver's detection threshold, can be ascertained by measuring the time that the detected burst exceeds the receiver's detection threshold. This information can be communicated to the external communication device. In response to receipt of such information regarding the relative strength of the detected downlink signals, the external communication device can modulate the peak amplitude of the downlink burst envelopes by modulating the gain of the external communication device transmitter. As such, the external communication device can then ensure an adequate margin over the implanted medical device's detection threshold while at the same time avoiding the transmission of unnecessarily high energy downlink signals.
Table 1 below lists U.S. Patents relating to telemetry systems.
TABLE 1 ______________________________________ U.S. Pat. No. Inventor(s) Issue Date ______________________________________ 4,211,235 Keller, Jr. 8 July 1980 4,374,382 Markowitz 15 February 1983 4,531,523 Anderson 30 July 1985 4,539,992 Calfee et al. 10 September 1985 4,550,732 Batty, Jr., et al. 5 November 1985 4,556,063 Thompson et al. 3 December 1985 4,571,589 Slocum et al. 18 February 1986 4,676,248 Berntson 30 June 1987 5,127,404 Wyborny et al. 7 July 1992 5,292,343 Blanchette et al. 8 March 1994 5,324,315 Grevious 28 June 1994 5,350,411 Ryan, et al. 27 September 1994 ______________________________________
All references listed in Table 1, and elsewhere herein, are incorporated by reference in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Embodiments, and claims set forth below, at least some of the devices and methods disclosed in the references of Table 1 and elsewhere herein may be modified advantageously by using the teachings of the present invention. However, the listing of any such references in Table 1, or elsewhere herein, is by no means an indication that such references are prior art to the present invention.